The portion of the electromagnetic spectrum ranging from about 0.3 THz to about 5 THz is often referred to as “the THz gap,” which reflects the fact that there are essentially no practical sources to produce continuous, tunable, narrowband radiation in this range. Despite over a decade of intense effort, the well-established emission techniques that have proven useful for the neighboring regions of the spectrum have proven ineffective for the THz band. However, this spectral region, because of its unique propagation characteristics and interaction with matter, holds the potential for exciting applications. Terahertz waves usually travel in line of sight. The radiation is non-ionizing, submillimeter microwave radiation and, like microwaves, has the capability to penetrate a wide variety of non-conducting materials. Terahertz radiation can pass through, for example, clothing, paper, cardboard, wood, masonry, plastic and ceramics. It can also penetrate fog and clouds, but does not penetrate metal or water. The Earth's atmosphere is a strong absorber of terahertz radiation which presents both advantages and challenges to its utilization.
A number of attempts have been made to produce practical and economically viable THz sources. Vacuum electronic devices, such as backward wave oscillators (BWOs), provide milliwatts of tunable power at high efficiency, but are not practical for portable applications due to their large size (and their need for a high-voltage supply and an external magnetic field). Despite their promise of high power, the same is true of all devices based on modulation of a high energy electron beam: their essential component is a large electron beam accelerator which cannot be made portable or lightweight with today's accelerator technologies. Quantum cascade lasers (QCL) have recently emerged as a new THz source, but these require complicated fabrication of quantum wells, and suffer from high power consumption, short lifetime (several hundred hours), and very low efficiency. Harmonic THz generation using microwave sources is a commercially available technology, but limited tunability and low efficiency remain a problem due to losses associated with harmonic conversion. In light of these limitations, Josephson junctions are a compelling alternative.
A Josephson junction is essentially two superconductors separated by a very thin insulating layer. A DC voltage applied across a properly designed junction causes it to oscillate and emit (or interact with) electromagnetic radiation. The emitted frequency is proportional to the applied voltage, up to a limit set by the superconducting energy gap. Investigation of Josephson Junctions as high frequency RF sources began as early as 1994, with the fabrication of an array that successfully functioned in the super-radiation regime at GHz frequencies. The total output power was about 10 μW and was delivered to a load rather than emitted into free space and detected as radiation. This effort involved using artificial Josephson Junctions (AJJs), which have several disadvantages. Because artificial junctions contain an amorphous insulating layer separating the superconducting layers, the crystalline properties of each layer can vary, causing each junction to emit at a different frequency. Furthermore, the small superconducting energy gap associated with AJJs limits the upper operating frequency to less than 1 THz and the large junction dimensions require that they be spaced 1 radiation wavelength apart in order to emit with the same phase. Such an arrangement limits emission to only discrete frequencies (i.e., limited tunability) and severely limits total output power, as power is proportional to the square of the total number of junctions. More recent efforts using Josephson Junctions has focused on establishing a “driven vortex lattice” in the superconducting material which, at the surface, creates an oscillating magnetic field. Metallic structures, such as Bragg gratings, are then placed on the surface causing the vortex lattice's field to radiate into free space. This approach does not create a laser-like interaction among Josephson junctions and is severely limited, therefore, in terms of output power, bandwidth, and tunability.
There exists a continuing need, therefore, for a THz radiation source that has increased and scalable output power, spans the broad range of THz radiation, is tunable to essentially any frequency within this range, and which is commercially viable.